Claire Petersky wrote:

Similarly, I don't think it's totally out of line for me to buy a small
package of say, Scottish Shortbread, announce to the family that it was
expressly purchased for my personal benefit

I do this part of this. But I'm more direct and self-serving: "I bought
this for me. It's mine. Lay off." (Okay, not those words.) Direct works
at our house.

and expect that there might be
a piece of shortbread in the package at any one time.

There is only one item, Luna bars, that I attempt to go this route with
(say that it's "mine" but The Kid can have some, within reason). The
inevitable result is always that my daughter will rifle through them
until there is only one or two left of the flavors I like the least and
she doesn't really care for. Then she will leave those "for me." In my
experience, expecting it to be otherwise is a fruitless venture into
self-aggravation.

Personally, I pick which strategy I use based on how badly I want to be
able to depend on something being there when I want it. If I wanted
guaranteed access to my shortbread (for example), I'd do one of two
things: I'd say "This is mine. Lay off." Or, if I was feeling more
generous, I'd buy two packages and label them (or hide one!). "This
package is mine. Lay off. This package is yours; you are welcome to it,
but when it is gone, it's gone."

I use the above practice especially with boxes of Girl Scout cookies,
since I go through cookies so much slower than my husband and daughter.
"These are your boxes." "This is my box. Keep out." I might choose to
share some of my box, but no one is allowed to break into it
independently of that. It works at my house. (Maybe it's because my
family is terrified of me when I'm unexpectedly out of thin mints.
). Your mileage may vary, unfortunately.

The main advantages to me and family: Expectations are fully known and
can be easily met without much thought.

Hope this helps. Good luck!

beeswing